New study: How do recent forest bird trends compare to changes in forest quality and quantity?

A team of Swedish ornithologists and forest experts compared temporal trends from two nationwide long-term monitoring schemes, the Swedish Bird Survey (1998-2015) and the Swedish National Forestry Inventory (1983-2014). Although this is “only” a national analysis, it includes highly representative values for both forest and bird changes over an area of 350 000 km2, and with a large latitudinal span from the Nemoral zone to the Arctic.

Since 1998 the total area of middle-aged and mature forest in Sweden has increased by 6.4%. In parallel, several forest structures potentially beneficial to birds (dead wood, retention trees on clear cuts, multi-layer forests, old forest and broadleaved forest) increased somewhat in abundance, most likely as a result of legislation changes and increasing areas under forest certification schemes. Summer temperatures also increased, with warm summers dominating since 2002. In 1998-2015, the population sizes of 58 forest bird species on average increased, as did the number of species observed per route, with no general difference between forest specialists (16 species) and generalists (42 species). However, from around 2005, the positive trends in bird numbers and many forest structures have levelled out.

An analysis of species population trends in relation to a measure of climate sensitivity (Species Temperature Index, STI) suggested that forest birds, just like Swedish birds in general, have indeed been affected by a warming climate. But given their STI, forest birds on average had more positive trends than non-forest birds, suggesting that other factors than climate have caused them to have relatively positive trends. Strong candidate factors are the documented changes in forest quality and quantity. While the analyses presented are purely correlational, and no firm conclusions on causality can be drawn, it is still reasonable to assume that larger areas of mature production forests, slightly improved forest quality, and warmer summers, all have contributed to the general increase in forest bird numbers in Sweden. But the relative contribution of these driving forces remains to be determined.

When it comes to the potentially positive effects of improving forest quality in terms of increases in old forest, stratification, retention trees and dead wood, it is noteworthy that, first, the forest quality in general is still below what is considered necessary for forest birds and other biodiversity to thrive, and second, several of the positive trends in forest structures since the mid-1990s seem to have ceased recently.